1. Field of the Invention
This invention relates to an article and to a process employing said article. More specifically, this invention involves a xeroprinting master suitable for use in a xeroprinting process.
2. Description of the Prior Art
The formation and development of images on an imaging layer of photoconductive materials by electrostatic means is well known. The best known of the commercial processes, more commonly known as xerography, involves forming a latent electrostatic image on the imaging layer of an imaging member by first uniformly electrostatically charging the surface of the imaging layer in the dark and then exposing this electrostatically charged surface to a light and shadow image. The light struck areas of the imaging layer are thus rendered relatively conductive and the electrostatic charge selectively dissipated in these irradiated areas. After the photoconductor is exposed, the latent electrostatic image on this image bearing surface is rendered visible by development with charged finely divided colored electroscopic powder material, known in the art as "toner". This toner will be principally attracted to those areas on the image bearing surface having a polarity opposite to the charge on said toner particles and thus form a visible powder image.
The developed image can then be read or permanently affixed to the photoconductor in the event that the imaging layer is not to be reused. This latter practice is usually followed with respect to the binder-type photoconductive films (e.g. zinc oxide pigment dispersed in a film forming insulating resin) where the photoconductive layer is also an integral part of the finished copy, U.S. Pat. Nos. 3,121,006 and 3,121,007.
In so-called "plain paper" copying systems, the latent image can be developed on the imaging surface of a reusable photoconductor or transferred to another surface, such as a sheet of paper, and thereafter developed. When the latent image is developed on the imaging surface of a reusable photoconductor, it is subsequently transferred to another substrate and then permanently affixed thereto. Any one of a variety of well-known techniques can be used to permanently affix the toner image to the copy sheet, including overcoating with transparent films and solvent or thermal fusion of the toner particles to the supportive substrate. It is not generally possible to prepare more than one copy of an original from a single latent image, since the latent image is partially neutralized by development and progressively decays with the passage of time between the development and transfer of each successive copy. This neutralization and decay results in a reduction in contrast potential between the latent image pattern and the non-imaged areas on the photoreceptor and thus a reduction in image density in each successive copy. The omission of a cleaning step in such a multiple reproduction copying system also results in further gradual deterioration in copy quality due to accumulation of toner residues on the non-imaged areas of the photoconductive layer.
In addition to the classical type of electrostatographic imaging systems discussed above wherein latent image formation takes place directly on the photoconductive insulating layer, the literature also discloses several processes wherein latent image formation results from inducement of a charge pattern across the dielectric overcoating, U.S. Pat. Nos. 3,234,019 (to Hall); 3,653,064 (to Inoue); and 3,676,117 (to Kinoshita). The ability of such induction imaging systems to prepare multiple copies of an original from a latent image is also limited by the same factors discussed above with regard to more conventional types of electrophotographic copying methods. It has been suggested, that where the latent image pattern can be isolated from the ambient environment and the charged electroscopic toner particles used in development of said image, such latent image can be used for preparation of multiple copies without repetition of the latent image formation sequence, U.S. Pat. No. 3,429,701 (to Koehler). However, without at least some periodic renewal of the charge pattern at the interface of the dielectric and the photoconductive insulating layer, the field intensity across the dielectric layer will gradually begin to decline as a consequence of development of each successive copy. This is due, in part, to the gradual build-up of a counter charge corresponding to the lines of force of the image pattern on the surface of the dielectric layer. As this countercharge increases, the contrast potential between the latent image pattern and the background areas of the dielectric layer will diminish to a point where it is no longer possible to effectively develop the image pattern.
The use of persistent photoconductors in electrostatographic imaging members and methods has been previously disclosed, U.S. Pat. No. 3,545,969. The persistent photoconductive imaging members disclosed in U.S. Pat. No. 3,545,969 are described as capable of preparation of multiple copies from a single exposure. This is reportedly achieved (column 6, line 46 to column 7, line 27) by initially exposing the persistent photoconductive imaging member to an image pattern followed by charging the exposed surface of said member. The charged surface of the persistent photoconductive imaging member is thereafter contacted with an insulating sheet, the surface of the sheet not in contact with the photoconductive imaging member also charged and the insulating sheet and the persistent photoconductive imaging member subsequently separated. Upon separation, a charge pattern is formed on the surface of the insulating sheet formerly in contact with the surface of the persistent photoconductive imaging member. This image pattern on the insulating sheet can be subsequently developed by standard techniques. The above imaging process can reportedly be repeated in excess of 100 times without reimaging of the persistent photoconductor. The quality of the image obtained by the above process is apparently dependent upon the type of charging means used in creation of a biasing potential on the surface of the insulating sheet not in contact with the persistent photoconductive imaging member. Where the means for charging the insulating member is a corona electrode, copy quality is marginal. In the event that charging of the insulating sheet is achieved with a conductive roller, copy quality is reportedly substantially improved. Upon separation of the insulating sheet from the persistently imaged photoconductive layer, air molecules at the point of separation are selectively ionized and thereby form a latent image pattern in the surface of the insulator in substantial conformity with the persistent image pattern. As a consequence of such ionization of the air molecules at the point of separation of these two films, the intensity of the image pattern within the persistently imaged photoconductive layer will be partially diminished. Such diminution in intensity of the persistent image pattern will be occasioned upon repetition of the copying cycle and thus ultimately the image information recorded within this layer will no longer be reproducible.
Accordingly, it is the object of this invention to remove the above as well as related deficiencies in the prior art.
More specifically, it is the principal object of this invention to provide a persistently photoconductive imaging member suitable for use in preparation of multiple copies from a single exposure to image information.
It is another object of this invention to provide a persistently photoconductive imaging member suitable for use in an induction type imaging system.
It is yet another object of this invention to provide an overcoated persistently photoconductive imaging member.